Recently, low bandgap photovoltaic cells such as the GaSb cell have made it possible to produce practical thermophotovoltaic (TPV) electric power generators. The low bandgap cells in these TPV generators convert infrared (IR) radiation from heated (IR) emitters into electric power. The IR emitters in these units operate at moderate temperatures between 900.degree. C. and 1400.degree. C. Baseline commercial TPV generators use gray-body SiC emitters with GaSb cells. The SiC emitter emits infrared energy at all wavelengths. However, the GaSb cells convert only infrared photons with wavelengths less than 1.8 microns to electric power. Infrared filters are used to reflect some of the non-useful longer wavelength photons back to the emitter. Unfortunately, the available filters are far from perfect. Some non-convertible infrared radiation still passes through the filters, and some of the reflected photons do not hit the emitter after reflection by the filter.
It is preferable to replace the gray-body emitter with a "matched" infrared emitter that emits only convertible infrared radiation. Mathematically, this perfect "matched" emitter has an emittance of 1.0 for wavelengths less than 1.8 microns and 0 for longer wavelengths. Several prior art infrared emitters have been proposed for use in TPV generators.
The oldest type of IR emitter proposed is the rare earth oxide selective emitter. Erbia is an example of this type of emitter. While the emittance at 1.5 microns can be as high as 0.5, the emittance for erbia falls to 0.1 at 1.4 and 1.6 microns and rises again beyond 3 microns. The result is that the emitted useful power is small because of the narrow emittance bandwidth. Furthermore, the spectral efficiency, defined as the in-band convertible power divided by the total emitted power, is low because a lot of power is emitted at wavelengths beyond 3 microns.
Refractory metal IR emitters, such as tungsten, have also been described. Those materials are somewhat selective in that the emittance at 1.5 microns (typically 0.3) is higher than the emittance at longer wavelengths (0.15 at 3 microns). Unlike the oxide emitters, the emittance stays low at long wavelengths (0.1 at 6 microns). Unfortunately, these metal emitters need to run very hot because of the low in-band emittance. They also produce volatile oxides when operated in air.
Recently, JX Crystals has described a cobalt doped spinel "matched" emitter. This "matched" emitter has an emittance of 0.7 at 1.5 microns with a large bandwidth. This emitter is selective, because the emittance falls off to 0.25 at 3 microns. Unfortunately, however, like all oxide emitters, the emittance rises again beyond 6 microns.
There are other disadvantages of the oxide emitters. Specifically, they are subject to cracking upon extensive thermal cycling, and they have poor thermal conductivity.
It is desirable to find an improved "matched" emitter with a high emittance at wavelengths below 1.8 microns and low emittance for all longer wavelengths. It is very desirable to find a "matched" emitter coating that may be applied to the current SiC emitter structures, since SiC is a proven material with good thermal conductivity and thermal cycle durability.